The present invention generally relates to an image detecting unit, and more particularly to an image detecting unit which is suited for use in an optical image reading device such as would be found in a facsimile, a copy machine and the like.
An optical image reading device has an image detecting unit. The image detecting unit includes a light emitting part which irradiates images on a medium and a light receiving part which receives a reflected light obtained by a reflection of the light emitted from the light emitting part on a surface of the medium.
Conventional image detecting units are disclosed in Japanese Laid-Open Patent Application Nos. 60-124165, 61-278265 and 63-299269. In each of these conventional image detecting units, a light beam emitted from the light emitting part is made thin and is slantingly incident on the surface of the medium so that the light emitted from the light emitting part is efficiently used for detecting an image.
An edge receiving type image detecting unit has been proposed. In the edge receiving type image detecting unit, the light receiving part has an optical waveguide comprising a core layer and a clad layer which surrounds the core layer. The reflected light from the medium is incident on an end surface of the optical waveguide, and then an incident light on the surface of the optical waveguide is transmitted through the optical waveguide to an optical detecting device.
A resolving power of the edge receiving type image detecting unit is determined on the basis of the critical angle of the optical waveguide and the distance between the surface of the medium (document) and the light receiving part of the edge receiving type image detecting unit. The critical angle of the optical waveguide is determined on the basis of refractive indexes of the core layer and the clad layer. The critical angle of the optical waveguide increases when the ratio between the refractive indexes of the core layer and the clad layer increases. When the critical angle increases, the amount of the light transmitted through the optical waveguide also increases, but the resolving power of the image detecting unit decreases. If there is a smaller distance between the document and the light receiving part becomes smaller, the resolving power of the image detecting unit is increased. However, when the distance between the document and the light receiving part becomes too small, it becomes difficult for the light beam from the light emitting part to be incident on the surface of the document. Therefore, the amount of the light transmitted through the optical waveguide becomes insufficient for detecting the image so that a signal-to-noise ratio of an image signal obtained by the light receiving part decreases. On the other hand, when the distance between the document and the light receiving part becomes too large, the resolving power and the amount of the light transmitted through the optical waveguide respectively decrease.
Accordingly, to obtain both a good resolving power and a sufficient amount of light transmitted through the optical waveguide, it is necessary to make the distance between the document and the light receiving part relatively small. In addition, it is also necessary for the light beam from the light emitting part to be thin and to be slantingly incident on an area, on the document, to which the end surface of the optical waveguide is directly opposed.
In conventional edge receiving type image detecting units, the light from the light emitting part is incident on the surface of the document in an arbitrary angle with regard to a normal direction of the surface of the document. That is, conventionally, the light from the light emitting part is slantingly incident on the surface of the document, but the optimum incident angle of the light beam to the surface of the document has not been yet examined. The incident angle of the light beam is defined as the angle between the light beam and the normal direction of the surface of the document. In addition, the relationship between the resolving power of the light receiving part and the distance between the light receiving part and the document has been not quantitatively studied.